son changed windows 10 username  

Even if you change a username in Windows, you should be able to sign in with the old password.

In screenshot I have changed my local account name Kari to Larry (#1). This does not change my user profile name as can be seen in folder C:\Users (#2), and in Advanced User Control Panel (#3):

This means that a username change in Windows is just superficial, cosmetic change with nothing to do with nor any effect in underlying user profile. When I now sign out I can see my new username in sign-in screen but continue signing in with password of the user profile, the original username:

I had both password and PIN set before the username change, both continue working with new username. A simple Windows username change does not lock user out, sign in continues to work with slightly changed credentials (username changed but password and PIN or picture password remain the same).

Kari

You simply log in to your son's user account using the original password, assuming he didn't change that, too (you told he changed username, nothing about password).

If no password was set, you sign in simply with Enter or clicking Sign in button.

There's nothing much you can do. If the laptop has a built-in recovery partition, consult the manual to see how to launch recovery to restore it to factory conditions, or if it came with a set of recovery DVDs use them.

If factory recovery is not possible, download Windows 10 ISO and clean install Windows. Notice that when clean installation asks for a product key you can skip it by selecting "I don't have a product key"; Windows will be automatically activated when it gets to desktop and is connected to Internet to check the digital signature of original installation.

Remember to download the same edition as the original, Home or Pro. If a general ISO containing both images has been downloaded, next dialog after skipping product key will ask if you want to install Home or Pro edition. Choose correct edition.

• Windows 10 ISO Download - Windows 10 Forums
• Windows 10 - Clean Install - Windows 10 Forums

   Tip

Windows 10 Anniversary Update will be released tomorrow, Microsoft has publicly announced they will make new ISO images available the same day.

I would wait until tomorrow evening before downloading ISO; downloading it now you have quite an upgrade process ahead when you have installed it.

Kari

NavyLCDR said:

Look for the tutorial on these forums on how to activate the built-in administrator account and use option number four from that tutorial. Then you can create a new administrator account or reset the old accounts password using the built-in administrator account.

You beat me to it Navy. :)
Administrator account - Enable or Disable in Windows 10 - Windows 10 Forums